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iiii)iNGs o:^ tHE Kotal Society, No. 172, 1876.] 
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Preliminary Note on the Use of the Piezometer in Deep- Sea 
Sounding.^"* By J. Y. Buchanan, Chemist to the ^ Challenger' 
Expedition. 

In order to determine the depth o£ the sea independently of the length 
of sounding-line used, piezometers filled with distilled water were fre- 
quently attached to the line along with the deep-sea thermometers. The 
combined effects of change of temperature and change of pressure were 
registered by a steel index of ordinary form. The temperature of the 
bottom-water being given by the deep-sea thermometer, the effect of 
temperature on the apparent volume of water in the piezometer could 
easily be calculated ; and from the residual effect, the pressure, and 
therefore the height, of the column of water to which the instrument 
had been subjected could be deduced. 

The piezometer did not differ materially from the ordinary ones used 
for the determination of the compressibility of liquids. A minute descrip- 
tion of the fittings necessary for their safe use on the sounding-line 
cannot be given without reference to a drawing or model, and must there- 
fore be postponed. 

It is manifest that if the apparent compressibility of water is accu- 
rately known, we shall be in a position to determine, by means of our 
instrument and a deep-sea thermometer, the depth to which it has been 
sunk, independently of the lengths of sounding-line used; for the indica- 
tions of the instrument depend solely on the temperature of the water at 
the depth in question, and on its vertical distance from the surface. 

The determination of the effect of change of temperature on such an 
instrument does not demand explanation. It is, however, otherwise mth 
the effects of pressure. In submitting an instrument of the kind to high 
pressures in an hydraulic machine, we encounter difficulty in accurately 
determining the pressure to which it is exposed, and also, although in a 
minor degree, in making our observations at the low temperature usually 
obtaining in deep ocean waters. I have therefore taken as basis for the 
determination of the apparent compressibility of water the results ob- 
tained when the instrument has been sent down on the sounding-line, 
either to the bottom or to intermediate depths, in positions where there 
has been no apparent disturbance from currents, and where the amounts 
of compression produced have been proportional to the depths recorded 
by the sounding-lines. Where currents are absent, and their presence is 
at once detected by the behaviour of the sounding-line, the depth, as 
determined according to the method of sounding adopted on board the 
' Challenger,' gives an excellent measure of the pressure exercised on the 
instruments. As the variations in the temperature, the salinity, and the 
compressibility of sea-wat-er with the depth have been thoroughly inves- 

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162 Mr. J. Y. Buchanan on the Use of [JUne 15, 

tigated for the soundings in question, the weight of a column of sea- 
water in any of these localities can be calculated with great accuracy. 

The observations which have been taken as a basis for determinations 
of depth were made in the latter part of the year 1875 in the South - 
Pacific Ocean. They were twenty in number, and were made at depths 
varying from 500 to 2300 fathoms, and at temperatures varyiug from 1^-4 
to 4°-03 C. The mean compressibility of water determined from these 
observations was 0-0008986 per 100 fathoms of sea-water, the extreme 
values being 0*000915 and 0-000882. Observations made at greater 
depths in the North Pacific gave as a mean of six observations at depths 
varying from 2740 to 3125 fathoms the value 0-000878, indicating a 
slight diminution in the coefficient of compression at very high pres- 
sures. 

The effect of pressure being thus known, we are in a position, by 
comparing the indications of the instrument with those of a trustworthy 
deep-sea thermometer, to determine the absolute depth to which it has 
been sunk beneath the surface ; and assuming the depth as indicated by 
the sounding-line to be correct, we should be able to determine the tem- 
perature at the depth in question from the indications of our instrument, 
and without the use of a thermometer. For the latter purpose, however, 
the instrument, as above described, is useless, because the dilatability of 
water at the low temperatiu'es obtaining in deep water is so small as to 
be negligible compared with its elasticity. 

The application, however, of the principle above indicated would mani- 
festly present some very great advantages in the determination of deep- 
sea temperatures. 

In the open ocean, where, as a rule, the temperature diminishes con- 
stantly as the depth increases,' the Millar-Casella thermometer gives 
sufficiently accurate results. In the case of enclosed seas, or in the 
neighbourhood of ice, however, this is not always the case. In the Medi- 
terranean, the Red Sea, and many of the seas of the Eastern archipelago, 
besides, possibly, large tracts both of the Atlantic and Pacific Oceans, the 
temperature decreases regularly down to a certain depth, which is different 
for different seas ; and at all greater depths the Millar-Casella thermo- 
meter gives identical readings, indicating that the water is either at the 
same temperature or some higher one. In the neighbourhood of ice, layers 
of water are frequently met with at various depths whose temperature, 
being higher than that of the surface, is indicated by the maximum index of 
the Millar-Casella thermometer. Besides these layers there may be, and 
there probably are, others whose temperature is higher than that of the 
water immediately above them \Aithout reaching that of the surface, and 
their temperature would remain unrecorded. It would therefore be of 
great advantage if the piezometer could be adapted for the determination 
of temperatures at known depths. An efficient instrument for this pur- 
pose has been obtained by filling the bulb of the piezometer with mercury 



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[1876. the Piezometer in Deep- Sea Sounding. 163 

'•) . 

r^ instead of water. The portion of the stem in which the index moves is 

filled with water, and, as in the other, the open end dips into a cup of 
mercury. We have thus an instrument filJe^I with a very large quantity 
, of mercury and a very small quantity of water; and after immersion the 
position of the index shows the apparent volume assumed by this mix- 
ture under the combined influence of temperature and pressure. As far 
as the eifects of temperature are concerned, the amount of water in the 
instrument is almost wholly negligible ; but when the effect of pressure is 
considered, the apparent compressibility of mercury is so small, being 
little more than one fiftieth of that of water, that the pj^ssure of even so pT-c-n-c^^^c^ 
small a quantity of water as can be contained in the graduated tube in- 
creases very materially the amount of contraction produced by pressure. 
The instrument, which has been in use since the beginning of November 
last year, and which is designated XVII. a, contains 2.56-61 grammes of 
mercury in the bulb and stem immediately above it; the volume of the "~ 

part of the stem filled ^Aih. water is 0-1935 c. c. The apparent contrac- 
tion of this mass of mercury and water is 0*000581 cubic centimetres per 
100 fathoms, and 0-0025 c. c. per degree respectively. A fall therefore 
of one degree in temperature produces the same effect as an increase of 
pressure equal to 430 fathoms of sea-water. Hence (and this forms the 
important peculiarity of the instrument) as long as the temperature of 
the sea does not increase with the depth at a greater rate than 1° C. per 
430 fathoms, the instrument will record the temperature correctly. The 
ratio subsisting between the amount of temperature and the column of 
water, which produce the same effect on the apparent volume, is a con- 
stant for every instrument ; in our one it is ^,j. By altering only very 
slightly the amount of water, the sensibility to pressure is greatly in- 
creased or diminished, while that to temperature remains practically 
unchanged. As the instrument XYII. a was intended principally for 
bottom-waters, the above ratio (jg-jj-) was considered sufficient, and it has 
proved practically useful. It must be remembered that the greater the 
value of this ratio is made, the greater is the error introduced into the 
determination of the temperature by any inaccuracy in the measurement 
of the depth. 

By attaching a combination of one, or better of two, of each of these 
instruments close to the weight at the end of the sounding-line, the 
depth of the sea and the temperature of the water at the bottom at any 
locality can be accurately determined, provided that sufficient evidence is 
afforded, either by the presence of a sample of bottom in the sounding- 
tube, or by the rate at which the line runs out, that the instruments have 
been at the bottom ; otherwise the depth which they have attained and 
the temperature at that depth will be correctly given. For this purpose 
it is necessary first to let the Line run out until, from observations on its 
velocity, it is evident that the weight has reached the bottom ; the length 

LC Control Niomber 




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164 Use of the Piezometer in Deep-Sea Sounding. [June 15, 

of line which has so run out will give the depth approximately, but more 
or less in excess of the truth according to circumstances. Allowing for 
the contraction which would be produced by this depth in the case of 
the mercury piezometer, a first approximation to the temperature of the 
bottom-water is at once obtained ; and it is sufficiently accurate for the 
purpose of correctly determining the contraction produced on the water 
piezometer by the change of temperature, and consequently for deducing 
the depth to which the instrument has been sunk. By now applying the 
more correct depth to the reading of the mercury instrument, we obtain 
the correct temperature, and if necessary the approximation might be 
carried still closer. 

As an example of the use of the combined instruments, the observa- 
tions made on the 29th February, 1876, may be taken. The position of 
the sounding was lat. 36° 9' S., long. 48° 22' W., and the depth by line 
was 2800 fathoms. The sea was quite calm, but there was a strong cur- 
rent setting to the south-east, rendering it probable that the depth, as 
determined by line, was considerably in excess of the true depth. The 
mercury instrument (XVIl. a) registered 166-2 millims. In order to 
clear this reading for a depth of 2800 fathoms, we have to subtract 16 
millims., and we obtain 150-2 millims. as the corrected reading, from which 
we determine the temperature to be -|- 0°*2 C. The reading of the water 
instrument was 283-8 millims. Assuming the temperature to have been 
0°*2 C, this would indicate that the water had suffered an apparent con- 
traction, owing to pressure alone, of 0'1923 c. c, which would be pro- 
duced by a column of 2480 fathoms of sea-water. Assuming now 2480 
fathoms to be the true depth, we find the corrected reading of the mer- 
cury instrument (XVII. a) to be 152*1 millims., which indicates a tem- 
perature of — 0°-5 C. The Millar-CaseUa thermometers gave the tempera- 
ture as — 0°-4. Assuming this as the correct bottom temperature, and 
reducing the reading of the water instrument (C. No. 1) accordingly, we 
find the contraction produced by pressure to be 0*1924 c.c, which agrees 
sensibly with that found on the assumption of the higher bottom-tem- 
perature of 4-0°-2 C. 

It will thus be seen that the two instruments fulfil the conditions re- 
quired of them; namely, that the one which is to indicate the temperature 
of the water shall be independent of great accuracy in the determination 
of the depth, and the one which is to indicate the depth shall be equally 
independent of accurate determination of the temperature; whilst by 
combining the results obtained by the two, an accurate determination is 
obtained both of the depth rand of the temperature of the water at that 
depth. 



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